Saw-tooth wave generator



Jan. 16, 1951 s. l. rouRsHou 2,538,541

SAWTOOTH WAVE GENERATOR Filed Sept. 27, 1947 ATTORNEY the targetelectrode.

Patented Jan. 16, 1951 UNITED STATES PATENT OFFICE SAW-TOOTH WAVEGENERATOR Simeon I. Tourshou, Philadelphia, Pa., assigner to RadioCorporation of America, a corporation of Delaware Application September27, 1947, Serial No. 776,465

3 Claims.

The present invention relates to generators for producing non-sinusoidalwaves, and more particularly, but .not necessarily exclusively, to suchgenerators for producing waves substantially of sawtooth form.

Where a beam of electrons is generated and directed against a targetelectrode, some l'means is normally provided for causing deflection ofthis cathode ray beam in two different directions in order that thecathode ray beam may be caused to scan or traverse the desired' portionof In a television camera or receiver, the electron beam is deflected ata slow speed, usually in the vertical direction, and at A Va high speed,usually in the horizontal direction.

The high speed deection provides line scanning and the low speeddeflection provides eld or frame scanning. In `oscillographic devices,the

cathode ray beam is usually deiiected horizontally at a predeterminedrate, while the beam is Viiected uniformly with respect to time, theapplied deiiection voltage must appear approximately as a sawtooth waveform or be of such shape as to produce a current of sawtooth wave form`in an inductance. In accordance with this invention, a recurringsawtooth of voltage, suitable for slow speed orhigh speed scanning, ,is

produced by a novel oscillator which is generally control the freerunning speed of the oscillator.

An lobject of this invention is to provide a sawtooth wave generatorconsisting of simple parts so that it is cheap to manufacture.

Another object is to provide a sawtooth wave 1 generator which does notrequire a transformer having a laminated iron core and separate primaryand secondary windings. A further object of the invention is to provid asawtooth wave generator, the free running speed of which mayy becontrolled in a novel manner by providing means for changing theinductance of a coil.

Other objects and advantages of the present invention will, of course,become apparent and immediately suggest themselves to those skilled inthe art to which the invention is directed from a reading of thefollowing specication in connection with the accompanying drawings inwhich:

Fig. 1 is a, diagrammatic illustration of a sawtooth wave generatorembodying the invention;

Fig. 2 shows a series of curves;

Fig. 3 shows a curve obtained during operation of the generator with adifferent degree of damping;

Fig. 4 shows a modification of the generator vof Figure 1; and

dit

Fig, 5 shows a further form .which is preferred -for certainAapplications.

Referring now to the drawing, and for the present to Fig. 1 thereof,there is shown a single coil i2, which may be regarded as anautotransformer having a secondary or grid section I4 and a primary orplate section It. The plate section i6 is connected to the anode I8 of adischarge tube, such as the three electrode vacuum tube I9. The grid 2lof this tube is connected through a capacitance 23 to the end of thegrid section It. synchronizing impulses, such as television syncsignals, usually in the form of timed recurlring pulses 26, are fed intothe cathode 28 of the tube i9. The cathode is returned to ground by way`of an unbypassed resistor 3 I.

A capacitance 32 is connected to a suitable vsource of positivepotential (not shown), which ,may be connected to a terminal 33 throughresistors 36 and 31 in series. Under certain conditions of operation ofthe generator of this invention, the capacitance 32 is charged throughthe resistors 31 and 36. Discharge, or partial discharge, of thiscapacitance is brought about by a sync pulse 26, which initiates anoscillation slightly before the would-be free-running oscillation which,as the grid swings positive, discharges er partly discharges thecapacitance 32. A resistor 38 serves as a damping resistor, and in Fig.l it is shown as being connected across the entire autotransformer coilI2. This resistor may be connected across any portion of the winding.

The output voltage of sawtooth wave 'form is taken from the resistor 36,which serves as a potentiometer having a, movable contact el. With thepotentiometer connection shown, rthe frequency of oscillation of thegenerator `is independent of the setting. A change in position which,

of the contact 4l provides forY variable high peaking so that the outputwave may havera variable impulse part.

In the operation of the arrangement of Figure 1, the capacitance 32 ischarged through the resistors 3S and 31 in series, the charge curve 44(Fig. 2) being substantially linear, initially. The sync pulses 26,shown as extending in the positive direction in Fig. 2 for convenience,bring about an abrupt discharge of the capacitance 32 as represented at46. is in the discharge path of the capacitance, as

Awell as in its charging pathhas the function of providing the pulseportion 48 of the Wave, since the capacitance 32 cannot be completelydischarged due to the time constant of the capacitance 32 and theresistor 3B as well as the plate resistance of the tube. When the tubei9 is blocked, as will be explained hereinafter, the plate Voltage doesnot have to rise slowly from the cathode potential, but instead risesimmediately to an appreciable value, due to the charge that remains onthe capacitance 32. The plate voltage then slowly rises along the curve44 asV indicated, is substantially linear. Upon the occurrence of a syncpulse 25, the grid voltage is raised immediately and increases in thepositive direction due to transformer action, as indicated at (Fig. 2).Following this, the grid is driven abruptly negative when thecapacitance 32 is nearly discharged and a negative charge is stored inthe capacitance 23, which leaks olf asIv represented by the portion ofthe curve designated 52.

The negative condition of the grid, which'is lassumed abruptly, asstated, is a result of several factors, the discharge of ythecapacitance 32 being only one of these factors.' The principal reasonfor the grid going negative isV the oscillatory circuit comprising theinductance andi distributed capacity of the coil I2, as well'as the tubeand stray capacity of the circuit, so that having reached the positivepeak, the voltage starts decreasing and then swings negative,

The sectending toY follow a damped sine wave. ond reason is the collapseof the plate current which induces a voltage in the coil tending toaccelerate the negative swing and even making it greater than positive.The third is the negative charge stored in the capacitance 23.

The discharge of the capacitance 23 occurs through a grid resistor 53 inseries with anadjustable resistor 54. The timgof discharge can beregulated by varying the resistance ofthe resistor 54. Just before theconduction point of the tube is reached, the occurrence of the nextsuccessive sync pulse 26 causes the cycle to be repeated. Fig. 3 of thedrawings shows the wave form when the degree of damping, whichisdetermined by the resistor 38, is less.

Fig. 4 of the drawings shows a slightly modied arrangement of theapparatus of Figure 1 in which the damping resistor, designated 38a,

bridges the plate section la of the coil, krather ,than the entire coil,as shown in Fig. l.

The damping resistor 38a may be connected across any portion o1 thewinding. A core 56 is also indicated, which may be of powdered and com-:the sync input is supplied to an auxiliary Wind- The resistor 36, whichlfrio l'ent is the following:

4 ing 58, which is coupled to the main winding 12b. The direction ofwinding of the auxiliary coil 58, or the connection of the sync input tothe terminals 6I takes care of the polarity of the applied sync signal.An adjustable core 62 is mounted in any suitable manner (not shown) sothatvit may be moved into or out of the coil, somewhat like the tuninginductance on a radio receiver. This change in inductance changes theresonant frequency and the impedance of the circuit, as Well as thecoupling between the plate and grid sections of the winding, and throughany one, or any combination of these changes, Varies the peak voltagedeveloped on the grid, as well as the plate. The frequency ofoscillation is varied since the charge on the capacitance is varied andthe time for this charge to leak off is varied. This method of controleliminates the cost of the variable resistor 54 of Fig. 1.

The windings I2, |20. and l2?) are preferably of the universal type withlow distributed ca- Dacity with parameters essentially independent of"voltage In general, this coil is more stable than the laminated ironcore transformer.. When the universal'type of coil is connected' as anautotransformen it maintains the ratio of plate currentY to grid currentin the positive grid region. The frequency is a function .of this ratio.By making the amplitude of oscillations and therefore the average biasdeveloped by oscillations, large enough, the effect of variation of thecontact potentialfof the tube is minimized.A

Having nowdescribed the invention what is claimed and desired to besecured by Letters Patl. A blocking oscillator comprising'a vacuumtubehaving a controll grid, a plate and a cathode, an autotransformercomprising two inductively coupled-coil sections,- one of said coilsections being connecteddirectly to the plate of said tube,

-a capacitor, a connection from the other of said coil sectionsV to saidgrid through said capacitor, means including a resistor connected. tothe junction of said coil sections to maintain said plate positive withrespect to said cathode, a second capacitor connected to the junctionlof said coil sections, said second capacitor also being eifectivelyconnected to the point of connection of the cathode in said oscillator,said second capacitor being charged'from said positive potential ap-,plying means and being discharged by said tube upon occurrence of apulse of grid voltage, and a damping resistor connected across at leastapart of said autotransformer.

2. The oscillator of claim 1, wherein said coil sections consist ofwindings having low distributed capacity.

3. A blocking oscillator comprising a vacuum tube having a control grid,a plate and a cathode, an autotransformer comprising two inductivelycoupled coil sections, one of said coil sections being connecteddirectly to the plate of said tube,

-a capacitorfa connection from the other of said coil sections to saidgrid through said capacitor, means including a resistor connected to thejunction of said coil sections to maintain said plate positive withrespect to said cathode, a second capacitor connected to the junction ofsaid coil sections through a second resistor,'said second capacitor alsobeing effectively connected to the -point of connection of the cathodein said oscillator, said second capacitor being charged from saidpositive potential applying means and being -discharged by said tubeupon occurrence of "a V5 `pu1se of grid voltage, and means for obtainingan output wave having a variable pulse component from said oscillator,said means comprising a connection to said second resistor which isadjustable along said second resistor. f

SIMEON I. TOURSHOU.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number 6 UNITED STATES PATENTS Name Date Smiley June 8, 1937 Faudell etal. Aug. 20, 1940 Smith Nov. 3, 1942 Lyman Oct. 17, 194A Gottier Apr.29, 1947

